FIELD OF THE INVENTION
The invention relates to a circuit configuration for receiving an on-off-keying-modulated signal (OOK-modulated signal), in particular for use in a data carrier of an identification system. The configuration has a reception circuit, a demodulator circuit which is connected downstream of the latter, and a decoding circuit which is connected downstream of the latter and a sequential controller for controlling in particular the reception, demodulation and processing of received data.
Such a circuit configuration is known from Published, European Patent Application EP 0 669 591 A2.
In the case of identification systems, data are exchanged between a stationary or quasi-stationary unit and a mobile or portable unit--referred to below as a data carrier--which has at least one memory. Power is also frequently transmitted from the stationary unit to the data carrier, preferably by inductive coupling or electromagnetic radiation. Examples of this are access control devices with a portable identifier, such as a chip card for example. However, electronic immobilizers with a key-lock system are also examples of the identification systems of the generic type.
In identification systems of the generic type, data are transmitted from the stationary unit to the data carrier by OOK-modulation. The high-frequency carrier signal is used directly as a clock signal--possibly after suitable division and/or conditioning for digital signal processing--with the result that the blanking intervals occurring in the carrier signal owing to the modulation also appear in the clock signal. In addition, the power that is necessary to supply the data carrier is acquired by rectification and smoothing of the transmitted signal.
The OOK-modulated carrier signal is demodulated either directly from the transmitted and received signal or from the clock signal that is acquired from the signal. This is possible because the clock signal also has the blanking intervals, and thus the modulation. The circuits in the data carrier of the known identification system are configured here in such a way that a brief gating out of the clock is tolerated.
The reception of data takes place here according to a predefined protocol that is controlled by a sequential controller (state machine). Interruption in the clock at a time other than that permitted according to the rules of the reception protocol causes the reception to be aborted.
Published, European Patent Application EP 0 387 071 A1 also discloses how a reset signal is derived in a transponder from the clock signal which has been derived from an OOK-modulated carrier signal. The reset signal resets circuit components of the transponder if the pause between two time periods in which the carrier signal occurs exceeds a specific time period.
After the reception of the data that may constitute instructions or else values that are to be processed, the data are processed. Such processing contains reading and writing data or values in the nonvolatile memory of the data carrier and also the transmission of data to the stationary unit, and possibly the encryption of data. During the processing of the received data, the data carrier does not expect any further data, and thus any interruption in the clock, because the carrier signal is not modulated by the stationary unit.
However, this leads to a security risk because in principle the system tolerates the clock being stopped. A hacker could then stop the clock, or gate it out, and examine the respective state of the circuit and thus find out about the structure and method of operation in order to enable himself to simulate it.
In order to counter this problem, there is already the tendency in the specialist world to use amplitude modulation with a relatively low modulation index instead of a 100% OOK modulation. Therefore, less robust demodulation has to be implemented on the data carrier, which leads to increased costs due to a lower yield, more complex measurement method or poorer compatibility.